I understand you want to know why you can’t post the data directly as a new data row in the linked table, like we can in some other apps, and have the text convert over as a new entry in the other table. If Coda allowed this it would break the data rather than keeping it “living” but maybe it will become a popular request…
In the world of good database management, best practices are to avoid at all costs manually manipulating the data, but instead make forms, and filters, or mark rows as “deleted” instead of actually deleting them. Over time this allows you as a database manager to have systematic control, keep organized, eliminate sparse data (rows with missing info), avoid manipulation of the data by bad code, and backtrack history. This prevents contractions and bugs.
So if you have a lookup table, it would be standard practice to make that entry in that second table, and then select it from the lookup list in the first. You can copy and paste the lookup value, which isn’t text anymore, but becomes an actual reference to the data. In a sense it’s alive.